Ruler of Planet Doom, King Zarkon controls a vast empire of enslaved worlds. The return of Voltron is a thorn in his side, gradually freeing worlds from his control and keeping Planet Arus out of his reach.
Called back to Planet Doom after Yurak's repeated failures to destory Voltron, the Crown Prince of Doom hasn't fared much better. Lotor is obsessed with the ruler of Planet Arus, Princess Allura. He has decided she will be his bride....by force if no other way works.
Yurak commanded the forces of Planet Doom when the space explorers first came to Arus and brought back Voltron. Defeated time and time again his "reward" for this was being replaced by Prince Lotor and turned into a robeast. Yurak met his fate at the end of Voltron's blazing sword.